Welcome! To answer your questions in order...
1) The ISF is very efficient, but it also depends on how you drive it. It's very easy to fill the tank only after 250 miles or so.
2) Cars, especially luxury cars take huge hit with depreciation. Mileage, age and condition are things to consider. Sure an ISF will have more value than a regular IS350 considering they are the same year with similar mileage.
3) Reliability is not an issue with Lexus. For example my 09 ISF which I bought new in December 08 has nearly 30k miles. If this car is going to be your daily driver, you may want to consider a few things. Cost of fuel usage per year, and your insurance rates. Because the ISF is considered a high performance sports car, rates will be higher than a regular IS350.
4) Squeaky brakes are issues with all brakes on cars, even more so with high performance brakes and big brake kits. All ISF owners will experience it at one point in time. Aggressive compound brake pads will eat up the rotors, especially if you drive hard. Brake pad replacement and rotors are not cheap on the F.
5) The ISF's V8 uses nearly 10 quarts of oil. 5k oil and filter change intervals are recommended by Lexus. You can choose to do the oil yourself or you can take it in to have it done. If you do a DIY synthetic oil change, you're looking at roughly $70.00 in oil, and about $13.00 for the OE oil filter. The tires that come stock on ISF's are summer high performance tires. Expected lifespan of these tires are 15k or less. Some have gotten well beyond that, myself included. It all depends on your driving habits. For example a replacement set of 4 Michelin PS2 tires for my ISF is roughly $1300 for just the tires alone.
The ISF is a fantastic car, but the maintenance costs can be high for replacement parts.